(1) Field of the Invention
The present invention relates to a process for producing optical waveguide devices to be used as second harmonic generation devices of a quasi-phase matching type and for use as optical modulators
(2) Related Art Statement
Devices which are produced by forming optical waveguides on a variety of materials to control light have been researched and developed, and such devices have been assembled into optical communication systems. In particular, a so-called ridge-shaped optical waveguide is expected as an optical modulator, optical switch element, etc. Further, a second harmonic generating (SHG) device of a quasi-phase matched (QPM) type using an optical waveguide made of lithium niobate or lithium tantalate single crystal in which a periodical domain inversion structure is formed has been. The second harmonic generation device may be used as an optical disc memory a variety of applications. For example, in medical applications, photochemical applications, various optical measurements, etc.
Optical waveguides have been formed in the optical waveguide type modulator or the SHG devices using lithium niobate, and quartz glass waveguides on silicon substrate bodies. In such cases, optical waveguides having a ridge-shaped structure have been formed by known etching methods such as RIE (reactive ion etching). More specifically, it is known that such a ridge-shaped structure may be formed by transferring a mask pattern on a substrate workpiece using photolithography, and then removing a portion of the optical waveguide-forming layer, other than the mask pattern, by ion etching or the like.
This process will be briefly outlined with reference to FIGS. 3(a) to 3(b). As shown in FIG. 3(a), an epitaxial film 22 is formed on a substrate body 21 made of an optoelectric single crystal, and a mask 23 having a given pattern is formed on a main plane 22a of the epitaxial film 22. Then, as shown in FIG. 3(b), a ridge-shaped optical waveguide 27 is formed by etching the epitaxial film 22. Since the epitaxial film 22 is removed up to a given depth, excluding a portion not masked, a resulting epitaxial film 24 has a reduced thickness as shown in FIG. 3(b).
However, according to this process, since high energy ions are irradiated upon the substrate body 21, the substrate body 21 is likely to be damaged so that a work damaged layer may be formed in the most important optical waveguide 27 through which light is to be passed. Since the thickness of the work damaged layer amounts to as much as around a few .mu.m, characteristics of the optical waveguide, for example, refractive index, change due to influences of the work damaged layer. As a result, there is a problem that actual characteristics of the optical waveguide device produced differ from those grasped in the simulation. Further, it has been discovered that since the stability of the optical waveguide decreases, an propagation loss and an extinction ratio of the optical waveguide unfavorably are deteriorated.
Further, since it takes a very long time to entirely and uniformly etch, for example, a 3-inch wafer at a depth of a few microns, this process becomes extremely high cost.
Further, since simulation results of the optical device calculated based on the refractive index, etc. of the material are not coincident with the structure of the optical device actually produced, the light absorption characteristic, the extinction ratio, etc. cannot be sufficiently attained. This is because a top face 26 of the ridge-shaped optical waveguide 27 is substantially flat, but side faces 28 are inclined.
The reason why the side faces 28 of the optical waveguide 27 are inclined is thought as follows. That is, the ridge-shaped optical waveguide 27 projects upwardly from the main plane 24a of the epitaxial film 24. In this case, in order to increase a ratio of d/W in which d and W are a height and a width of the ridge-shaped optical waveguide 27, respectively, to project the ridge-shaped optical waveguide 27 at a large height, it is necessary to etch a surrounding portion of the ridge-shaped optical waveguide 27 as deep as possible. However, since a ratio in etched degree between the substrate body and the mask is ordinarily in a range of 2:1 to 5:1, a mask 23 having a correspondingly increased thickness needs to be used so as to deeply etch the surrounding portion of the ridge-shaped optical waveguide 27. If such a thick mask 23 is used, the etched rate around the mask then decreases, so that a ridge angle ".theta." becomes extremely smaller than 90.degree..